The present invention relates generally to waterborne coatings. More particularly, the present invention relates to acrylic-based binder systems for waterborne coatings and the waterborne coatings produced from such binder systems.
The emphasis on environmental protection together with increasingly restrictive legislation has led to the increased use of coatings based upon water rather than organic solvents.
Although waterborne primer/surfacers may now be considered normal state-of-the-art coatings in the automobile industry, color topcoats continue to represent a major source of organic solvent pollution. When it is considered that current, conventional, solid-color topcoats contain 50% or more organic solvents and that some 5 kg of paint are required to coat a compact car, it is clear that the total solvent emission from this source is substantial.
The increasingly popular clear-over-base metallic finishes exacerbate this problem, with the basecoat alone presently accounting for 50% of total emissions from the paint line. Such basecoats may contain up to 88% solvent and require a final clear coating which itself may have 50% or more organic solvent content.
Considerable work has, therefore, been undertaken to replace these solvent-based coatings with higher solids and waterborne systems, and major efforts have been concentrated on the metallic basecoats. Various systems have been proposed and examples are described in EP-A-228 142, EP-A-238 222, EP-A-242 731, EP-A-251 921 and U.S. Pat. No. 4,213,886, all of which are hereby incorporated by reference. Conventional clearcoats or two-component clearcoats with higher solid contents (60-65%) are proposed as final coatings for these basecoats.
The use of conventional organic solvent-based topcoats with the hydrobasecoats, however, leads to problems. In the "wet-in-wet" application of the solvent-based topcoat to the hydro-basecoat, for example, appearance defects due to "strike-in" phenomena may be produced. To minimize this, an expensive and time consuming pre-dry step must be utilized for the basecoat. Additionally, organic solvents utilized in the conventional topcoats remain a source of pollution.
It would, therefore, be highly desirable to produce a waterborne coating which finds utility, in part, as a topcoat for hydro-basecoats. Additionally, such waterborne coating should desirably have physical and mechanical properties comparable to conventional coating materials, should be easy to apply (i.e., using the wet-in-wet method), and should not exhibit "strike-in" phenomena when so applied. It should also possess good pigment/extender wetting and dispersion properties for use in pigmented forms such as surfacers, solid-color topcoats and the like.
Waterborne coatings produced from water-soluble acrylic and/or polyester resin binder systems have been described in the literature and utilized commercially, but only with limited success as topcoats for hydro-basecoats. As topcoats, they generally exhibit unfavorable rheology, and require the use of large amounts of amines or cosolvents to lower viscosity sufficiently for reliable application and to ensure stability of the system. This, however, results in poor appearance (solvent "strike-") and also in a low solids content which makes it difficult to reach the required film thickness with normal application methods.
More recently, waterborne topcoats have been produced from "hybrid" binder systems comprising combinations of water-soluble reins with aqueous dispersion resins. An example is the combination of Synthacryl 6483 (a commercial acrylic dispersion) with Synthacryl 6484 (a commercial water soluble acrylic resin), which is recommended by the Hoechst AG as a water-based clearcoat for two-coat metallic finishes. Other examples are described in U.S. Pat. Nos. 3,919,154; 3,953,644; 4,518,724; and 4,624,973, all of which are hereby incorporated by reference. The resulting waterborne coatings display somewhat improved properties, but rheology and low solids content problems remain. Also, the waterborne coatings utilizing these hybrid binder systems normally still require large amounts (20-25%) of cosolvents, resulting in a topcoat with less than ideal appearance characteristics, sagging on application and a tendency to solvent-popping on drying.